Core-shell catalyst and method for palladium-based core particle

ABSTRACT

A core-shell catalyst includes a porous, palladium-based core particle and a catalytic layer on the particle. The particle can be made by providing a precursor particle that has palladium interspersed with a sacrificial material. At least a portion of the sacrificial material is then removed such that the remaining precursor particle is porous.

BACKGROUND

This disclosure relates to a high activity catalyst for use in electrochemical or other catalytic devices.

Electrochemical devices are commonly used for generating electric current. For example, an electrochemical device may include an anode catalyst, a cathode catalyst, and an electrolyte between the anode and cathode catalyst, for generating an electric current in a known electrochemical reaction between reactants. One issue encountered with electrochemical devices is the operational efficiency of the catalyst. For example, electrochemical activity at the cathode catalyst is one parameter that controls the efficiency. An indication of the electrochemical activity is the rate of electrochemical reduction of the reactant at the cathode catalyst. Platinum has been used for the catalyst. For example, platinum has been alloyed with other metals to produce ternary or quaternary alloys with enhanced catalytic activity and stability. Platinum has also been dispersed on core particles to form what is known as a core-shell catalyst.

SUMMARY

A core-shell catalyst includes a porous, palladium-based core particle and a catalytic layer on the particle. The particle can be made by providing a precursor particle that has palladium interspersed with a sacrificial material. At least a portion of the sacrificial material is then removed such that the remaining precursor particle is porous.

Also disclosed is a method of making a core-shell catalyst. The method includes identifying a target surface area for a core-shell catalyst and forming a porous, palladium-based core particle in response to the target surface area such that the porous, palladium-based core particle has the target surface area. A catalytic layer is then deposited on the porous, palladium-based core particle to form the core-shell catalyst with the target surface area. The higher the surface area, the more the platinum can be deposited on the core. A high fuel cell performance requires a higher platinum loading and low transition metal loading for a core-shell catalyst. The surface area and amount of transition metal can be well controlled by the disclosed method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an example core-shell catalyst.

FIG. 1B shows a TEM image of a portion of a core-shell catalyst.

FIG. 2 shows an example method of making a porous, palladium-based core particle.

FIG. 3 shows an example method of making a core-shell catalyst.

DETAILED DESCRIPTION

FIG. 1A schematically shows an example core-shell catalyst 20 and FIG. 1B shows a TEM image of a portion of the core-shell catalyst 20. As can be appreciated, the core-shell catalyst 20 can be used in electrochemical devices or other devices that would benefit therefrom. As will be described in further detail, the core-shell catalyst 20 includes a structure and materials that provide enhanced electrochemical activity.

The core-shell catalyst 20 includes a porous, palladium-based core particle 22 (“particle 22”) and a catalytic layer 24 on the particle 22. The porosity of the particle 22 provides a greater amount of free surface area for the catalytic layer 24, which may infiltrate into the particle 22 and coat the internal surfaces thereof. The particle 22 includes a solid, continuous lattice structure 26 (represented by the white portions) and an open porosity 28. The open porosity 28 extends throughout the bulk of the particle 22. In one example, the particle 22 has a porosity of 20% or greater. In another example, the particle 22 has a porosity of 50% or greater.

The particle 22 is palladium-based and therefore includes palladium as the major metallic constituent. In one example, the particle 22 has only palladium, to the exclusion of other metals in greater than trace impurity amounts. In another example, the particle 22 includes palladium as the major metallic constituent and at least one transition metal. The transition metal or metals can include any element in groups 3-12 of the Periodic Table, actinide elements and lanthanide elements. Optionally, the particles 22 can also include carbon-based materials, such as carbon black.

The amount and type of transition metal that is used serves to modify the atomic lattice structure of the palladium, which in turn influences the electrochemical activity of the core-shell catalyst 20. In one example, the transition metal includes nickel. In a further example, the particle 22 includes only palladium and nickel, to the exclusion of other elements in greater than trace impurity amounts.

The composition of the catalyst layer 24 can be selected for the intended end use of the core-shell catalyst 20. In one example, the catalyst layer 24 includes platinum, but alternatively or additionally can also include other metals. In a further example, the catalyst layer 24 is a platinum monolayer. A monolayer is approximately one atom thick. However, although the monolayer is generally of uniform thickness, it is to be understood that some portions may be thinner (submonolayer) or thicker (several atoms thick).

In a further example, the atomic lattice structure of the palladium of the particle 22 is modified by the addition of the one or more transition metals to enhance the electrochemical activity of the catalyst layer 24. In one example, the particle 22 has a ratio, by weight, of the amount of palladium to the amount of the one or more transition metals of 10:1 or greater. In a further example based on using platinum as the catalyst layer 24 and nickel as the transition metal in the particle 22, the ratio by weight of palladium to nickel is 20:1 or greater or is 30:1 or greater.

FIG. 2 shows an example method 40 of making the porous, palladium-based core particle 22 as described herein. The method 40 includes at step 42 providing a precursor particle that has palladium interspersed with a sacrificial material. At step 44, at least a portion of the sacrificial material is removed such that the remaining precursor particle is porous (i.e., the porous, palladium-based core particle 22). For example, the sacrificial material is removed by acid washing using an acid that has a greater reactivity with the sacrificial material than with the palladium.

In a further example, the sacrificial material is the one or more transition metals of the particle 22. For instance, the precursor particle includes the palladium and the one or more transition metals. The one or more transition metals are provided in the precursor particle in an amount that is greater than the desired amount in the particle 22. A portion of the one or more transition metals are then removed (i.e., “dealloyed”) to create the porosity in the particle 22. Thus, by volume, the initial amount of the one or more transition metals in the precursor particle is also greater than the created porosity because a portion of the one or more transition metals remains as a lattice-modifying agent in the particle 22 after the removal step 44 is completed.

In further examples, the method 40 utilizes either of two methodologies to provide the precursor particle, including an impregnation technique and a co-reduction technique. In the impregnation technique, solid palladium particles are mixed with a salt of the one or more transition metals. The salt is then reduced such that the one or more transition metals impregnate into the solid palladium particles to form the palladium interspersed with the one or more transition metals. At least the portion of the transition metal that is impregnated into the solid palladium particles is then removed to form the particle 22.

Alternatively, in the co-reduction technique, a palladium salt and a salt of the one or more transition metals are co-reduced to form the palladium interspersed with the one or more transition metals before then removing at least a portion of the transition metals to form the particle 22. In either of the techniques, impregnation or co-reduction, the precursor particle can be subjected to a heat treatment to facilitate interspersion of the palladium and the sacrificial material. The following examples further illustrate details of each of the techniques.

EXAMPLE 1 Impregnation Technique; 60° C. 1 M HNO₃ Acid Washing Procedure for PdNi₆/C

1) Add 1 g of 35% Pd/C to a 100 mL beaker with 50 mL ultrapure water, ultrasonicate and stir for 30 min

2) Dissolve the required amount of nickel nitrate hexahydrate in 25 mL water to achieve 1:6 molar ratio of Pd:Ni and add the precursor solution to the Pd/C dispersion

3) Stir the suspension on a stir plat and heat it to 95° C. until most of water evaporates to form a dense slurry or moist solids

4) Dry in a vacuum oven at 80° C. for a couple of days

5) Scrape the powder from beaker and grind with mortar and pestle and allow it to stand overnight in desiccator

6) Place 2 g of the powder in a ceramic or quartz boat and put it into a tube furnace

7) Heat to 400° C. in 2 hours in 4% H₂/argon and hold for 1 hour in argon

8) Heat to 700° C. in 1 hour in argon and hold for 4 hours in argon, and allow to cool to room temperature

9) Grind the catalyst (PdNi₆/C) with mortar and pestle

Acid Washing Step 1:

1) Add 800 mg PdNi₆/C to a 250 mL beaker with 230 mL ultrapure water, ultrasonicate and stir for 30 min; Transfer the PdNi₆/C suspension to a 500 mL round bottomed flask

2) Place the flask in a 60° C. water bath and keep stirring at 500 rpm

3) Add 172 mL preheated 60° C. 2.3 M HNO₃ into the flask quickly and keep stifling for 1.58 hours

4) Filter out the solids

Acid Washing Step 2:

1) Re-disperse the solids in 170 mL ultrapure water, ultrasonicate and stir for 30 min

2) Transfer the solids to a 250 mL round bottomed flask

3) Place the flask in a 60° C. water bath and keep stirring at 500 rpm

4) Add 47 mL preheated 60° C. 0.5 M H₂SO₄ into the flask quickly and keep stifling for 4-9 hours

5) Filter out the solids and wash the solids five times with 400 mL portions of ultrapure water

6) Dry the solids in a vacuum oven at 60° C. overnight; Grind the catalyst with mortar and pestle

7) Usually example 1 results in a porous core with Pd to Ni weight ratio about 20-30; Perform additional dealloying 60° C. 0.5 M H₂SO₄ as needed based on composition from ICP results if Ni content is too high

To generate a porous structure, the first step in the acid washing is to create pores using diluted HNO₃ solution at various temperatures. The higher the temperature, the shorter the time needed. The second step is to further remove Ni from the core using diluted H₂SO₄ at various temperatures. If only H₂SO₄ is used in both steps, the porous structure is not formed, resulting in a low surface area.

EXAMPLE 2 50° C. 1 M HNO₃ Acid Washing Procedure for PdNi₆/C from the Impregnation Method

1) Add 800 mg PdNi₆/C to a 250 mL beaker with 230 mL ultrapure water, ultrasonicate and stir for 30 min; Transfer the PdNi₆/C suspension to a 500 mL round bottomed flask

2) Place the flask in a 50° C. water bath and keep stirring at 500 rpm

3) Add 172 mL preheated 50° C. 2.3 M HNO₃ into the flask quickly and keep stifling for 5 hours

4) Filter out the solids

Acid Washing Step 2:

1) Re-disperse the solids in 170 mL ultrapure water, ultrasonicate and stir for 30 min

2) Transfer the solids to a 500 mL round bottomed flask

3) Place the flask in a 50° C. water bath and keep stirring at 500 rpm

4) Add 180 mL preheated 50° C. 1 M HNO₃ saturated with Pd(NO₃)₂ into the flask quickly and keep stirring for 8.5 hours

5) Filter out the solids and wash the solids five times with 400 mL portions of ultrapure water

6) Dry the solids in a vacuum oven at 60° C. overnight; Grind the catalyst with mortar and pestle

7) Usually example 2 results in a porous core with Pd to Ni weight ratio about 20-30; Perform additional dealloying of step 2 as needed based on composition from ICP results if Ni content is too high

EXAMPLE 3 Co-Reduction Technique; 80° C. 0.5 M H₂SO₄ Acid Washing Procedure for PdNi₆/C from the Co-reduction Method

1) Add 600 mg sieved Ketjen Black carbon support to a 1 L beaker in 600 mL ultrapure water, ultrasonicate and stir for 30 min

2) Add 1 g palladium nitrate hydrate and 6.56 g nickel nitrate hexahydrate while stifling at 400 RPM

3) Add 14.47 g PVP slowly to achieve a PVP-to-metal molar ratio of 0.1:1 (adjust stirring rate as needed to dissolve the PVP) and stir for 30 minutes

4) Add NaBH₄ solution (5.97 g in 100 mL water) dropwise into the mixture

5) Stir for 1 hour after adding the NaBH₄ solution at 800 RPM

6) Filter the solution and wash with 3 L of ultrapure water

7) Dry solids under a vacuum at 60° C. overnight or until dry

8) Passivate the solids using≈1-2% O₂/N₂ stream to prevent burning of the catalyst after it cools down (to avoid burning carbon)

9) Grind up solids once dry and record total weigh

10) Heat the catalyst to 700° C. in 1 hour and hold for 4 hours in argon

11) Cool to room temperature and grind the catalyst with mortar and pestle

Acid washing step 1:

1) Add 1.25 g PdNi₆/C to a 600 mL beaker with 490 mL ultrapure water, ultrasonicate and stir for 30 min; Transfer the PdNi₆/C suspension to a 500 mL round bottomed flask

2) Place the flask in a 80° C. water bath and keep stirring at 500 rpm

3) Add 135 mL preheated 80° C. 2.3 M H₂SO₄ into the flask quickly and keep stirring for 5 hours

4) Filter out the solids

Acid Washing Step 2:

1) Re-disperse the solids in 196 mL ultrapure water, ultrasonicate and stir for 30 min

2) Transfer the solids to a 500 mL round bottomed flask

3) Place the flask in a 80° C. water bath and keep stirring at 500 rpm

4) Add 47 mL preheated 80° C. 0.5 M H₂SO₄ into the flask quickly and keep stifling for 1.5-2 hours

5) Filter out the solids and wash the solids five times with 400 mL portions of ultrapure water

6) Dry the solids in a vacuum oven at 60° C. overnight; Grind the catalyst with mortar and pestle

7) Usually example 3 results in a porous core with Pd to Ni weight ratio about 20-30. Perform additional dealloying 80° C. 0.5 M H₂SO₄ as needed based on composition from ICP results if Ni content is too high

EXAMPLE 4 25° C. 1 M HNO₃ Acid Washing Procedure for PdNi₆/C from the Co-reduction Method

Acid washing step 1:

1) Add 1.25 g PdNi₆/C to a 600 mL beaker with 490 mL ultrapure water, ultrasonicate and stir for 30 min; Transfer the PdNi₆/C suspension to a 500 mL round bottomed flask

2) Add 135 mL 2.3 M HNO₃ into the flask quickly and keep stirring for 6.5 hours

3) Filter out the solids

Acid Washing Step 2:

1) Re-disperse the solids in 196 mL ultrapure water, ultrasonicate and stir for 30 min

2) Transfer the solids to a 500 mL round bottomed flask

3) Place the flask in a 80° C. water bath and keep stirring at 500 rpm

4) Add 47 mL preheated 80° C. 0.5 M H₂SO₄ into the flask quickly and keep stifling for 2-2.5 hours

5) Filter out the solids and wash the solids five times with 400 mL portions of ultrapure water

6) Dry the solids in a vacuum oven at 60° C. overnight; Grind the catalyst with mortar and pestle

7) Usually example 4 results in a porous core with Pd to Ni weight ratio about 20-30. Perform additional dealloying 80° C. 0.5 M H₂SO₄ as needed based on composition from ICP results if Ni content is too high

The Table below presents a summary of the materials, synthesis method, formation technique and resulting electrochemical area of the particle 22 in comparison to a reference catalyst particle of palladium deposited onto a carbon support. In the Table, “ICP (%)” refers to the composition by weight percentage as determined by a known inductively coupled plasma technique, where the balance of the composition is carbon-based material. From the table, it is clear that If only H₂SO₄ is used in both steps for the samples prepared from impregnation method, the porous structure is not formed resulting in a low surface area.

TABLE Experimental Summary Electrochemical Alloy synthesis Protocol surface area Materials Methods 1st step ICP(%) 2nd Step ICP (%) m²/g (Pd) PdNi6/C Impregnation 0.5M H₂SO₄, Pd: 32 0.5M H₂SO₄, Pd: 38 22 (not 80° C., 5 hour Ni: 15 80° C., 4 hour Ni: 1.2 porous) 1M HNO₃, Pd: 33 0.5M H₂SO₄, Pd: 35 39 50° C., 5 hour Ni: 12 80° C., 5 hour Ni: 1.1 1M HNO₃, Pd: 33 saturated Pd: 39 55 50° C., 5 hour Ni: 12 Pd(NO₃)2, 1M Ni: 1.5 HNO₃, 50° C., 8.5 hour 1M HNO₃, Pd: 30 0.5M H₂SO₄, Pd: 38 50 60° C., 1.58 hour Ni: 29 60° C., 9 hour Ni: 1.2 Co-reduction 0.5M H₂SO₄, Pd: 31 0.5M H₂SO₄, Pd: 26 40 80° C., 5 hour Ni: 8 80° C., 1.5-2 hour Ni: 1.2 1M HNO₃, Pd: 28 0.5M H₂SO₄, Pd: 22 50 20° C., 6.5 hour Ni: 5 80° C., 2-2.5 hour Ni: 1.1 1M HNO₃, Pd: 26-29, saturated Pd: 27 60 20° C., 6.5 hour Ni: 4-6 Pd(NO₃)2, 1M Ni: 1.5 HNO₃, 20° C., 8 hour Reference N/A N/A Pd: 33 N/A Pd: 33 62 (not Pd/C porous)

FIG. 3 illustrates a further example method 60 of making a core-shell catalyst 20. As can be appreciated from the prior described method 40, the amount of sacrificial material used in the precursor particle can be adjusted to provide a particle 22 with a desired, target surface area. The method 60 includes at step 62 identifying a target surface area for the core-shell catalyst 20. For example, the target surface area may vary depending upon the intended end use of the core-shell catalyst 20. At step 64 the porous, palladium-based core particle 22 is formed in response to the identification of the target surface area such that the particle 22 has the target surface area. At step 66, the catalytic layer 24 is deposited on the particle 22 to thereby form the core-shell catalyst 20 with the target surface are. Techniques for depositing such catalytic layers on core particles are known and the details are therefore not set forth in this disclosure.

Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.

The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims. 

What is claimed is:
 1. A core-shell catalyst comprising: a porous, palladium-based core particle; and a catalytic layer on the particle.
 2. The core-shell catalyst as recited in claim 1, wherein the porous, palladium-based core particle includes at least one transition metal.
 3. The core-shell catalyst as recited in claim 2, wherein the porous, palladium-based core particle has a ratio, by weight, of an amount of palladium to an amount of the at least one transition metal that is 10:1 or greater.
 4. The core-shell catalyst as recited in claim 3, wherein the ratio is 20:1 or greater.
 5. The core-shell catalyst as recited in claim 3, wherein the ratio is 30:1 or greater.
 6. The core-shell catalyst as recited in claim 2, wherein the at least one transition metal includes nickel.
 7. The core-shell catalyst as recited in claim 2, wherein the at least one transition metal titanium, vanadium, chromium, manganese, cobalt, iron, copper, zinc and molybdenum.
 8. The core-shell catalyst as recited in claim 1, wherein the porous, palladium-based core particle has a surface area of 20 meters squared per gram or greater.
 9. The core-shell catalyst as recited in claim 1, wherein the porous, palladium-based core particle has a surface area of 30 meters squared per gram or greater.
 10. The core-shell catalyst as recited in claim 1, wherein the porous, palladium-based core particle has a surface area of 50 meters squared per gram or greater.
 11. The core-shell catalyst as recited in claim 1, wherein the catalytic layer is a platinum monolayer.
 12. The core-shell catalyst as recited in claim 1, wherein the porous, palladium-based core particle is internally porous.
 13. The core-shell catalyst as recited in claim 1, wherein the porous, palladium-based core particle has a porosity of 20% or greater.
 14. A method of making a palladium-based core particle for a core-shell catalyst, the method comprising: (a) providing a precursor particle that has palladium interspersed with a sacrificial material; and (b) removing at least a portion of the sacrificial material such that the remaining precursor particle is porous.
 15. The method as recited in claim 14, wherein the sacrificial material includes at least one transition metal.
 16. The method as recited in claim 15, wherein the at least one transition metal includes nickel.
 17. The core-shell catalyst as recited in claim 15, wherein the at least one transition metal titanium, vanadium, chromium, manganese, cobalt, iron, copper, zinc and molybdenum.
 18. The method as recited in claim 14, further including selecting an amount of the sacrificial material in the precursor particle in response to a target, desired amount of porosity in the remaining precursor particle.
 19. The method as recited in claim 18, wherein the amount, by volume, of the sacrificial material in the precursor particle is greater than the target, desired amount of porosity in the remaining precursor particle such that a portion of the sacrificial material remains in the precursor particle after the removing step is completed.
 20. The method as recited in claim 14, wherein said step (a) includes providing a non-porous palladium particle and infusing the non-porous palladium particle with the sacrificial material.
 21. The method as recited in claim 14, wherein said step (a) includes providing a palladium salt and a salt of the sacrificial material, and reducing the palladium salt and the salt of the sacrificial material to form the precursor particle.
 22. The method as recited in claim 14, wherein said step (a) includes subjecting the precursor particle to a heat treatment to intersperse the palladium and the sacrificial material.
 23. The method as recited in claim 14, wherein said step (b) includes using an acid to create the pores and remove transition metals.
 24. The method as recited in claim 23, wherein the acid is selected from the group consisting of nitric acid, sulfuric acid, prechloric acid and acetic acid.
 25. The method as recited in claim 23, wherein step (b) is conducted at a temperature of up to 90° C.
 26. The method as recited in claim 23, including two or more steps of acid washing.
 27. A method of making a core-shell catalyst, the method comprising: (a) identifying a target surface area for a core-shell catalyst; (b) forming a porous, palladium-based core particle in response to said step (a) such that the porous, palladium-based core particle has the target surface area; and (c) depositing a catalyst layer on the porous, palladium-based core particle to form the core-shell catalyst with the target surface area. 